1. Field of the Invention
This invention relates to an image processing apparatus, an image processing method and computer program product for combining PDL (page description language) image data and bitmap image data.
2. Description of the Related Art
Recent image forming apparatuses include functionality of both a copier and a printer. The apparatus is able to form images by scanning an original to obtain pixel-based image data directly (like a copier) and is also able to form images by interpreting commands in a page description language (“PDL”) so as to render the PDL commands into pixel-based image data (like a printer). PDL commands are not pixel-based images, whereas an image based on rendered PDL commands is a pixel-based image.
Such apparatuses are also able to form combined images, where part of a single printed image is obtained from scanning and another part is obtained from rendered PDL commands. The following is a discussion related to high quality reproduction for a combined image including a scanned image and an image based on rendered PDL commands. High quality reproduction processing for a combined image can be realized by changing color converting method RGB to YMCK, gamma correction method or binarization method. Preferably, processing is eased by processing the combined image without considering whether the processed pixel is a portion of a scanned image or a portion of an imaged based on rendered PDL commands.
As an example of high quality reproduction processing, copying machines have a special process for black characters which detects black characters portions in a scanned image and reproduces the detected black character portion by just black ink without using yellow ink, cyan ink, magenta ink. Detection of black characters must be performed on a pixel based image (a scanned image or a bitmap image) since in the detection of black characters, it is necessary to compare a pixel with its surrounding pixels. A printer interpreter renders PDL commands received via an interface as a bitmap image and prints the bitmap image. After rendering the PDL commands, the bitmap image is a pixel-based image just like the scanned image so detection of black character can be performed for the bitmap image. Then, the reproducing process for black characters can be performed to a portion of the image base on rendered PDL commands exactly the same as a portion of the scanned image. As a result, the reproducing process for black character can be performed without considering whether the processed pixel is a portion of the scanned image or a portion of the image based on rendered PDL commands.
However, since the detecting of black characters is a conjecture that the processed pixel is a part of a black character by comparing the processed pixel with its surrounding pixels, the detection results are not necessarily correct. If the process for black characters is performed based on the wrong detection result, the reproduced image is degraded.
Attributes of an image based on PDL commands can be detected perfectly by discriminating the kind of the PDL commands even without rendering PDL commands. The attributes discriminated from the PDL commands indicate whether the image portions based on the PDL commands are graphic portions, natural image portions or character portions. The attributes of the graphic portions, the nature portions or the character portions are different from definition based on the detecting of the black character in copying machines. The attributes and the definition of the detecting black character can not be used together without considering whether the processed pixel is a portion of the scanned image or a portion of an image based on PDL commands.